1. Technical Field
The present disclosure relates to a toner, a method of manufacturing toner, an image forming method, an image forming apparatus, and a process cartridge.
2. Description of Related Art
Recently, various high-speed and high-quality full-color image forming technologies have been developed in electrophotography. Japanese Patent Application Publication Nos. 07-209952 and 2000-075551 each describe a tandem image forming apparatus employing an intermediate transfer method. The apparatus includes tandemly-arranged multiple electrophotographic photoreceptors each adapted to form single-color toner image and an intermediate transfer member onto which the single-color toner images are transferred from the multiple electrophotographic photoreceptors. Intermediate transfer methods are advantageous in preventing the occurrence of background fouling but disadvantageous in terms of transfer efficiency because of including two transfer processes, i.e., the primary transfer process in which toner images are transferred from electrophotographic photoreceptors onto an intermediate transfer member and the secondary transfer process in which the toner images are further transferred from the intermediate transfer member onto a recording medium.
To meet demand for high-quality image, toners have been developed to have a much smaller particle size to more precisely reproduce latent images. Japanese Patent No. 3640918 and Japanese Patent Application Publication No. 06-250439 each describe a polymerization method for manufacturing toner. It is described therein that the polymerization method is capable of control particle size and shape of toner. It is also described therein that small-sized toner particles can precisely reproduce dots and thin lines with smaller pile height (i.e., image thickness).
Generally, in electrophotography, as the size of a toner particle gets smaller, non-electrostatic adhesive force between the toner particle and a photoreceptor or an intermediate transfer member gets larger, which results in poor transfer efficiency. Therefore, when small-sized toner particles are used in a high-speed full-color image forming apparatus, the secondary transfer efficiency may drastically decrease. The reason is not only that the non-electrostatic adhesive force between a toner particle and an intermediate transfer member is increased but also that a time period within which toner particles, in a multilayer, are exposed to the secondary transfer electric field is shortened in the high-speed apparatus, as the size of the toner particle gets smaller.
One approach for improving transfer efficiency includes increasing the secondary transfer electric field, but this approach not always improves transfer efficiency. Another approach for improving transfer efficiency includes increasing the width of the secondary transfer nip so that toner particles can be exposed to the secondary transfer electric field much longer. In a case in which a contact bias roller is employed in the secondary transfer, the contacting pressure or diameter of the bias roller may be increased to increase the width of the secondary transfer nip, each of which is not preferable in terms of image quality and compactness. In a case in which a non-contact charger is employed in the secondary transfer, the number of chargers may be increased to increase the width of the secondary transfer nip, which is not preferable in terms of compactness and cost.
Japanese Patent Application Publication No. 2001-066820 and Japanese Patent No. 3692829 each propose a technique for adjusting the kinds and amounts of external additives of toner. It is described therein that non-electrostatic adhesive force between a toner particle and a photoreceptor or an intermediate transfer member is reduced by adjusting the kinds and amounts of external additives.
However, the external additives may be gradually buried in toner particles as the toner particles receive mechanical stress due to agitation in a developing device. As a result, the external additives may no more function and transfer efficiency may deteriorate. This phenomenon notably occurs in high-speed apparatuses because agitation in developing device is more intensive.
One approach for providing high transfer efficiency for an extended period of time even in high-speed apparatuses involves controlling mechanical strength of the surfaces of toner particles so that external additives are not buried in the toner particles even under exposure to mechanical stress. The toner manufacturing method described in Japanese Patent No. 3640918 has attempted to form a thick layer of resin particles on the surfaces of toner particles to increase mechanical strength of the toner particles. Toner particles having too stiff surface cannot melt well when fused on a recording medium upon application of heat. In a case in which toner particles include a release agent such as wax, the release agent cannot exude from the toner particles when fused on a recording medium upon application of heat.